Using confocal microscopy on preparations of intact rat coronary artery (CA), we have observed that CA smooth muscle cells in situ generate frequent Ca2+ sparks from multiple discharging sites. In order to examine the SR Ca2+ releases and associated currents in more detail, an enzymatic isolation procedure was employed to produce isolated cells. Ca2+ sparks could still be observed in these cells following isolation. Cells were patch clamped and whole cell outward current measured. In all cells, a large sustained outward current was observed that exhibited oscillatory behaviour at very positive potentials. We have shown that approximately 70% of this whole cell current is mediated by voltage-dependent delayed rectifier K+ channels (Kv) and large conductance Ca2+-dependent K+ (BKCa) channels. Spontaneous transient outward currents (STOCs) were observed in these cells, the amplitudes of which were voltage-dependent. At 0 mV, STOCs attained amplitudes of up to 250 pA, whereas at -40 mV, amplitudes of 5-40 pA were observed. At very negative potentials (-80 mV), STOCs were no longer generated, but small spontaneous transient inward currents (STICs: 5-30 pA) were observed instead, which exhibited sensitivity to the Ca2+-activated Cl- channel (ClCa) blocker, niflumic acid (10 μM). Using current clamp conditions, the resting membrane potential was determined to be -41.6 ± 2.5 mV (n = 25). The effect of ion channel inhibition on membrane potential was examined and the influence of resting membrane potential on the generation of spontaneous currents, in response to SR Ca2+ releases, was studied. Membrane potential was shown to be dependent on Kv, BKCa and ClCa channels. The response to the generation of Ca2+ sparks by 1 mM caffeine exhibited membrane potential dependence, with STOCs occurring when membrane potential was relatively depolarised and STICs occurring when membrane potential was relatively hyperpolarised. At a holding potential of -40 mV, similar to the physiological resting membrane potential, spontaneously produced Ca2+ sparks generated STOCs in almost all cells (17/18 cells); however, STICs were also observed on some occasions (4/18 cells). To our knowledge, this is the first time that both STOCs and STICs have been observed in coronary artery smooth muscle cells and both events were shown to be highly membrane potential sensitive.